College Answer

a) List the possible causes for his respiratory failure.

Differential diagnosis should include:
 ARDS secondary to sepsis from any source or other inflammatory insult including the following
 Pneumonia (hospital-acquired)
 Aspiration
 Atelectasis/pleural effusions/empyema
 Fluid overload secondary to resuscitation, renal failure
 Exacerbation of pre-existing condition e.g. heart failure, valvular heart disease, post-op
ischaemia/MI, arrhythmia
 Lung diseases e.g. lymphangitis carcinomatosis

The patient is intubated and mechanical ventilatory support is initiated.

b) Describe the ventilator settings you will prescribe, giving the rationale for your decision.

 Use a mode with which one is familiar and aim to limit ventilator-associated lung injury, i.e
oxygen toxicity, barotrauma, volutrauma, shear stress and biotrauma
 Choice of mode (any appropriate answer acceptable e.g. APRV for recruitment benefit, or
volume assist control as staff familiarity and no one mode shown to have benefit over another)
 Avoid over-distention of alveoli by keeping tidal volumes at 6-8 ml/kg (predicted body weight
which in the ARDSnet studies was ~20% below actual body weight and calculated by a formula
linking height and sex)
 Use PEEP to minimise alveolar collapse and derecruitment.
 Titrate PEEP to achieve a PaO2 of 60 mmHg with lowest FiO2 that is needed using decremental
PEEP trial post recruitment manoeuvre.
 I:E ratio of  1:1
 Permissive hypercapnea to avoid large minute volumes and alveolar injury through collapse and expansion of lung units

Following intubation, there is no immediate improvement in the patient’s oxygenation

c) List the initial strategies that may be used to improve oxygenation.

 High FiO2 (titrated to lowest possible level to limit toxicity)
 Confirm ETT position and patency
 Exclude readily reversible cause of hypoxia e.g. PTX, mucus plug, large effusion
 Increased inspiratory time
 Increased PEEP
 Recruitment manoeuvre with decremental PEEP trial
 Prone positioning for at least 16/24 hours per day
 Ensure adequate cardiac output

Discussion

a) List the possible causes for his respiratory failure.

In the chapter on the definition, causes and differential diagnosis of ARDS there is this table:

The next question asks the candidate to describe the ventilator settings you will prescribe, giving the rationale for your decision.  After those settings fail to work, the college asks for strategies that may be used to improve oxygenation. Overall this question is asking about  Ventilation strategies for ARDS. These are discussed in greater detail elsewhere. A brief summary may be offered:

Initial ventilator strategy:

• Use a Pressure Control mode (it may be safer, though the evidence is not strong)
• Lung-protective ventilation: use low tidal volumes (6ml/kg)
• Open-lung ventilation: avoid derecruitment by using a high PEEP
  • The ideal PEEP can be found either by finding the lower inflection point or the pressure-volume curve or by observing a stepwise decrease in PEEP after a recruitment manoeuvre.
  • As the ARDS severity increases, consider using a higher PEEP. 
• Use a lower driving pressure (ΔP) -Amato et al, 2015. That means, using a higher PEEP and aiming for a lower plateau pressure
• Accept a level of "permissive hypercapnea"

Additional ventilator manoeuvres to improve oxygenation:

• Use an I:E ratio of 1:1, even though manipulating the I:E ratio does not seem to improve survival, even though it may improve oxygenation.
• One might attempt some recruitment manoeuvres if hemodynamics permit. Again, these offer a transient improvement in oxygenation, but do not influence survival.

Non-ventilator adjunctive therapies for ARDS:

Ventilator strategies to manage refractory hypoxia

• Prone ventilation, for at least 16 hours a day (PROSEVA, 2013)
• High frequency oscillatory ventilation may not improve mortality among all-comers (OSCAR, 2013) or it may actually increase mortality (OSCILLATE, 2013) but some authors feel that there were problems with methodology.

Non-ventilator adjuncts to manage refractory hypoxia

• Nitric oxide was a cause for some excitement, but is no longer recommended.
• Prostacyclin is still a cause for excitement, and is still vaguely recommended.
  • Neither agent improves mortality, but prostacyclin can improve oxygenation.
• ECMO may improve survival (CESAR, 2009) but again there were problems with methodology.